1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display panel including organic EL elements, which are electric light-emitting elements, and in particular to a technical improvement in an ejection process using an inkjet head.
2. Description of the Related Art
Organic EL elements, which have come into practical use in recent years, are current-driven light-emitting elements. Each organic EL element has the following basic structure. A light-emitting layer that includes an organic light-emitting material is disposed between a pair of electrodes, i.e., an anode and a cathode. When driven, voltage is impressed between the pair of electrodes, and holes injected into the light-emitting layer from the anode recombine with electrons injected into the light-emitting layer from the cathode. Each organic EL element makes use of the phenomenon of electroluminescence occurring during this recombination. Since each organic EL element emits light, the light from the organic EL display panel is highly visible. Furthermore, each organic EL element is a complete solid state device and thus has excellent impact resistance and the like.
Among different types of organic EL elements, one known type of organic EL element contains a light-emitting layer and a charge injection layer formed by ejecting ink containing a macromolecule material or small molecules that are suitable for thin-film formation in a wet process (ejection process) of, for example, an inkjet method. This type of organic EL element is called an ejection-type organic EL element. There are organic EL display panels that include, as units of light emission, a plurality of such ejection-type organic EL elements arranged in a matrix on a substrate. Such organic EL display panels have been commercialized as displays in small electronic devices, image display devices, and the like.
In a typical inkjet method (see Patent Literature 1), the substrate on which ink is to be ejected (hereinafter, “ejection target substrate”) is first placed on a work table. While the inkjet head is moved across the substrate in one of row and column directions, drops of ink are ejected through nozzles into the regions for forming the organic EL elements as defined by a bank on the substrate. Herein, the ink is a solution containing an organic material for the light-emitting layer or charge injection layer, as well as solvent. The inkjet head is driven by a driving method such as a piezoelectric method. The bank takes many shapes. One example is a line bank whose shape corresponds to a plurality of regions for forming the organic EL elements. However, a commonly-used example is a pixel bank that positionally defines each individual organic EL element.
When manufacturing an organic EL panel with a plurality of organic EL elements arranged therein, it is necessary to ensure that the characteristics of the organic EL elements are uniform. Therefore, it is required to eject a uniform amount of ink into each of the regions for forming the organic EL elements, so as to ensure that the light-emitting layer and the charge injection layer, which are formed by drying the ejected ink by evaporation, have a uniform thickness in each of the regions for forming the organic EL elements. For this reason, voltage applied to each nozzle is normally adjusted in the ejection process so that each nozzle ejects the ink at a constant speed (hereinafter, “ink ejection speed”). This method is employed to eject a uniform amount of ink into each of the regions for forming the organic EL elements on the substrate.